"Ian Stirling" wrote: What's wrong with simply using a bar bent into a
circle? (clip)
^^^^^^^^^^^
These are available on the market. Check with Craft Supplies, Packard, or
any other vendor selling wood lathe tools. The cover a wide price range,
depending on the diameter of the wood being handled, and the number of
coring radii provided.

Of course, if you built your own lathe, you may also prefer to build your
own coring system, but you do not need to re-invent the wheel. BTW, none of
them use spherical shaped cutters--it ain't necessary, as Ian pointed out.